Implantable cardiac stimulation devices, such as pacemakers and implantable cardioverter-defibrillators, are devices that are implanted within the body of a patient so as to correct and regulate heart function. Typically, these types of devices include one or more leads that are adapted to be implanted within the body of the patient so as to be adjacent the heart in order to deliver therapeutic electrical stimulation pulse to the heart. Further, these devices generally also include a control unit having a processor, that is positioned within a casing that is also adapted to be implanted within the body of the patient. Over time, the control units of implantable cardiac devices have become increasingly sophisticated thereby allowing the control units to tailor the therapeutic electrical stimulation that is provided to the heart to optimize device performance and heart regulation.
For example, current generation pacemakers are capable of detecting intrinsic heart activity so that the pacemaker only provides pacing pulses when the intrinsic heart activity is missing. Moreover, these types of more sophisticated implantable cardiac devices also incorporate numerous sensors which provide data to the control unit enabling the control unit to optimize the therapeutic stimulation provided to the heart.
Implantable cardiac devices also generally include telemetry circuits that allow a treating physician to download instructions to the control unit following implantation. Further, the telemetry circuit can also be used to allow the treating physician to retrieve stored information about heart activity and device performance. The treating physician may periodically want to review device performance or heart activity data to ensure that the device is providing therapy in desired manner. Consequently, current generation implantable cardiac devices incorporate memories and the processors in these devices are adapted to periodically sample and record various performance parameter measurements in the storage means.
One difficulty, however, is that implantable cardiac devices have significant limitations on both space and battery power which affect the ability of the processor to record data for subsequent review. Specifically, the space within the casing containing the control unit is usually at a premium as the overall size of the implanted casing is preferably kept to as small as possible to minimize patient discomfort. The casing must be able to accommodate the processor and the circuitry that produces the therapeutic waveform so that there is not much space that can accommodate memory devices for storing data.
Moreover, the sampling and recording of device or heart performance data also consumes power from the battery which reduces the longevity of the cardiac device. The more power that is consumed in the sampling and recording of device or heart data results in less power being available in the long term to provide needed therapeutic stimulation to the heart, necessitating earlier replacement of the pacemaker.
A still further effect of repetitive sampling and recording of device or heart performance data is the consumption of available random access memory (RAM), which may eventually impact pacemaker function. Space limitation in pacemaker housings gives rise to limitations on sizing of memory areas which, therefore, places design demands for the efficient use of available memory.
As a consequence, implantable cardiac devices are generally set to sample and then record device or heart parameters at periodic intervals. The periodic sampling of device or heart parameters reduces the overall drain on the battery particularly when the parameter is sampled by providing a test pulse to the heart. The periodic recording of the device data reduces the overall size requirements of the memory and further reduces the consumption of battery power during the recording process. However, if the selected parameter is sampled and recorded at a fixed frequency, and the performance of the device or heart changes during the interval between the recording of the parameter, important data relating the change in the parameter may be lost.
For example, one parameter that is periodically sampled and recorded by prior art pacemakers is capture threshold. Specifically, the pacemaker may be adapted to periodically provide a series of pacing pulses of decreasing magnitude to determine at what magnitude the delivered pacing pulse fails to induce a paced beat response. The last magnitude value that resulted in a paced beat response is known as the capture threshold. The capture threshold parameter can be used to set the magnitude of the pacing pulse to be delivered to the heart at some safety margin above the capture threshold to ensure the delivered pacing pulse induces a paced beat response of the heart. In autocapture pacing devices, the output is typically set to a value which is much closer to the threshold value, since more frequent capture threshold measurements are continuously being made to ensure that the output will be adequate to obtain capture even as changes in the capture threshold varies, which may be indicative of a developing problem with the lead or alterations in the clinical status of the patient. The capture thresholds are often recorded in the memory for subsequent review and analysis by the treating physician to detect these changes in threshold value which, as noted above, may be indicative of lead and implantation conditions.
In some devices, the capture threshold device parameter is measured on a relatively frequent basis for autocapture type devices, e.g., once or twice a day, and the resulting measurement is recorded in memory on a somewhat less frequent periodic basis, e.g., once or twice a week. However, it is understood that the capture threshold device parameter changes over time.
For example, immediately following implantation, the capture threshold is usually relatively low as the lead is positioned immediately adjacent the inner wall of the heart. However, inflammation of the heart tissue surrounding the implanted lead then begins to increase the impedance at the implantation site which results in the capture threshold increasing in magnitude. Subsequently, a fibrous tissue forms around the implanted lead adjacent the inner wall of the heart which results in an increase in the effective area of the electrode that is delivering the pacing pulse to the heart. This membrane results in a decrease in the capture threshold such that the capture threshold essentially stabilizes at a particular value.
The growth of the fibrous tissue and the resulting change in the capture threshold generally occurs over a one- to two-month period and is typically referred to as the period of lead maturation. Similarly, the lower, relatively stable, value of the capture threshold following the acute phase will typically persist, in the absence of any lead implantation problem, for an extended period of time. This period of relatively stable capture threshold is typically referred to as the chronic phase of lead implantation.
It is desirable to have a relatively considerable amount of data during the lead maturation so that the treating physician can monitor whether the change in the capture threshold indicates that the leads have been properly implanted. However, once the acute phase has ended, the need to sample and record the capture threshold data with heightened frequency is reduced and such heightened frequency recording can result in a significant drain on the battery and can also result in the memory being filled by less valuable data. Further, it is possible that more valuable data may even be overwritten by less valuable data. It is also desirable to allow available RAM to be more efficiently used to report periods of stability but to also report periods of instability in far greater detail than would be accomplished with a fixed preset sampling rate.
In many prior art devices, the treating physician can adjust the sampling and recording frequency of the device parameter using the telemetry circuit.
However, the patient may not return to the treating physician at an appropriate time for the adjustment of the sampling and recording frequency. Moreover, there may be a sudden change in the implanted device which may result in a significant change in the device or heart parameter. For example, if a lead becomes broken or partially dislodged, the capture threshold may vary significantly from the typical chronic phase value.
Other conditions may arise that would not result in a subsequent operative intervention, but would be managed totally by automatic or manual programming of the system. These would include, for example, capture threshold changes due to lead-tissue interface problems as may occur with progression of a patient's intrinsic disease process, concomitant diseases, such as kidney failure, which will cause the serum potassium level to rise, which will cause the capture threshold to rise, or the administration of a medication, which may be newly known to affect capture. Accordingly, if the treating physician has set the implanted cardiac device to record the device parameter at a lower frequency, critical data relating to the change in the device or heart parameter may not be captured.
Hence, there is a need for an implantable cardiac device that is capable of recording device or heart parameter data where the sampling and recording frequency can be varied by the device depending upon the relative change in the measured parameter. To this end, there is a need for an implanted cardiac device, such as a pacemaker, that is capable of measuring a parameter, such as capture threshold, and sampling and recording this particular parameter at a higher frequency when the parameter measurements are more volatile.